In many existing non-dispersive fluorescent x-ray analyzers, sequential comparative radiation measurements are necessary, perhaps with "balanced filters" to achieve adequate energy resolution. In the comparative measurement technique, substances of known and unknown compositions are irradiated. Fluorescent x-ray radiation emanating therefrom is measured in sequential operations of the radiation analyzer, so that the measurement results of the substance of unknown composition can be determined with respect to the measurement results of the substance of known composition. However, variations in measuring technique, instrument gain, and voltage fluctuations significantly affect the results obtained.
Balanced filters together transmit a narrow band of x-ray energies such that transmission by each filter of a filter pair outside of this band is "balanced," but transmission within the band is significantly different. The difference in the signal transmitted through the filters is proportional to the incident radiation intensity in the narrow band of unbalance. This narrow band of unbalance can be chosen to include the characteristic fluorescent radiation energy for an element of interest, and is typically less than 5 KeV in width. The balanced filter technique permits x-ray intensity measurements to be made with substantially higher energy resolution than the inherent resolution of the detector. The energy resolution is governed by the pass band of the filter pair rather than the inherent properties of the detector.
Stated another way, the term balanced filters, as used herein, refers to a pair of filters having different transmission properties in a narrow energy band of interest but having transmission properties closely matched for all other energies of interest. The quantitative determination of the amount of an element of interest is determined by the difference of the transmitted signals passed by the filters. One filter, hereinafter termed the "transmitting" filter, passes a greater portion of the radiation in a narrow energy range of interest than does the other filter, hereinafter termed the "absorbing" filter.
Several types of balanced filter analyzers are currently in use. One type sequentially interposes each of a pair of filters between the substance to be analyzed and a scintillator during a measuring cycle. A radiation count is taken for a predetermined time with one of the filters interposed between the detector and the specimen to be analyzed. The count is recorded and the other filter is placed in position, with radiation counts then being recorded for an equal time interval. The background radiation is nominally equivalent in both of the measuring intervals, but the extent by which the radiation count with the transmitting filter exceeds the radiation count with the absorbing filter is indicative of the extent to which the element of interest is present.
In other types of balanced filter analyzers, dual radiation detectors or dissimilar scintillators are utilized, one with the transmitting filter and the other with the absorbing filter. The counts from the two detectors are compared to determine the amount of radiation attributable to the elements of interest. In the known dissimilar scintillator system the respective signals are separated by rise time techniques. This differential count is indicative of the quantity of the element which is present, as previously discussed.
A treatment of balanced filters generally may be found in the book "X-Ray Analysis Papers", edited by William Parish, Centrex Publishing Company, Eindoven, 1965, at pages 36 and 37.